Adsorption vessels

ABSTRACT

The present invention relates to a filtration unit containing pellets or granules of fine-particle iron oxides and/or iron oxyhydroxides with a large specific surface area, processes for their production and processes for their use in the filtration units.

BACKGROUND OF THE INVENTION

[0001] The invention relates to filtration unit suitable for thethrough-flow of a fluid medium for the removal of a contaminant from thefluid medium, like an adsorption vessel through which a liquid to betreated can flow, particularly a filter adsorption vessel which, whenfilled with granulated or powdered, solid, water-insoluble adsorptionmedia, particularly iron (oxy)hydroxide, is used for the removal ofarsenic or heavy metals from drinking water. The device can be connectedto the sanitary and drinking water supply in the home, for example.

[0002] The invention also concerns a process for the production of theiron (oxy)hydroxide adsorbents for use in the filtration units accordingto the invention.

[0003] In 1999, studies by the National Academy of Science verified thatarsenic in drinking water causes bladder, lung and skin cancer.

[0004] A commonly occurring problem, particularly in regions wherespring water, tap water or drinking water in general is contaminatedwith arsenic or other heavy metals, is that there is no suitabledrinking water treatment plant nearby or no suitable device to hand thatwould continuously remove the contaminants.

[0005] Filter cartridges for cleaning liquids, preferably contaminatedwater, which can also contain an adsorption medium, are known in variousforms.

[0006] For example, membrane filter cartridges in suitable housings areused to separate solids from water.

[0007] Cartridges and devices for the treatment of liquids are knownfrom Brita Wasser-Filter-Systeme GmbH (DE-A 19 905 601; DE-A 19 915 829;DE-A 19 814 008, DE-A 19 615 102, DE-A 4 304 536, U.S. Pat. No.6,099,728). These devices are very suitable for the entire or partialremoval of salts from drinking water in domestic jugs immediately beforethe drinking water is used.

[0008] A filtration unit in the form of a filter cartridge having a bedof activated carbon particles between a polyester urethane foam layerand a glass-fibre layer is known from U.S. Pat. No. 4,064,876.

[0009] DE-A 19 816 871 (Sartorius) describes a filtration unit for theremoval of contaminants from fluids.

[0010] RU-A 2 027 676 describes a cartridge filter with sorption agentfor drinking water purification with a connection to the water tap inthe home.

[0011] HU-A 00 209 500 describes a filter cartridge for the removal ofradioactive material and heavy metals from water, which is filled with amixture of ion-exchange material, activated carbon, filter sand,zeolites, aluminium oxide and red mud.

[0012] These adsorber cartridges are generally filled with activatedcarbon or ion-exchange resins. The disadvantage of activated carbon,however, is that due to the low adsorption capacity of activated carbon,arsenic and heavy metal salts occurring in aqueous systems are notremoved to an adequate extent, and this has an effect on the servicelife of the cartridges.

[0013] The disadvantage of ion-exchange resins is that they are veryunselective in the way that they bind ions from aqueous solution, andcompetitive reactions commonly occur in the adsorption. A furtherdisadvantage of ion exchangers is that the adsorption capacity of theion exchanger is extremely dependent on the pH value of the water, suchthat large quantities of chemicals are needed to adjust the pH of thewater, which is not practicable when the adsorber cartridge is used inthe home.

[0014] Contact and adsorbent granules, including those based on ironoxides and/or iron oxyhydroxides, have already been described. They arepredominantly used in continuous processes, whereby they areconventionally found in tower or column-type units through which themedium to be treated flows, and the chemical or physical reaction oradsorption processes take place at the outer and inner surface of thegranules. Powdered materials cannot be used for this purpose becausethey compact in the direction of flow of the medium, thereby increasingthe flow resistance until the unit becomes blocked. If a unit is cleanedby back-flushing (see below), large amounts of the powder are dischargedand lost or cause an unacceptable contamination of the waste water.

[0015] The flowing media also exert forces on the granules, however,which can lead to abrasion and/or movement through to violent agitationof the granules. Consequently the granules collide, leading toundesirable abrasion. This results in loss of contact or adsorbentmaterial and contamination of the medium to be treated.

[0016] In gas purification the agent is used in adsorbers for bindingundesirable components such as hydrogen sulfide, mercaptans and hydrogencyanide, as well as other phosphorus, arsenic, antimony, sulfur,selenium, tellurium, cyano and heavy metal compounds in waste gases.Gases such as HF, HCl, H₂S, SO_(x), NO_(x) can also be adsorbed.

[0017] A filter cartridge for drying gases is described e.g. in U.S.Pat. No. 5,110,330.

[0018] The removal of phosphorus, arsenic, antimony, selenium,tellurium, cyano and heavy metal compounds from waste oils and othercontaminated organic solvents is also possible.

[0019] Contact and adsorbent granules based on iron oxides and/or ironoxyhydroxides are also used for the catalysis of chemical reactions inthe gas phase or in the liquid phase.

[0020] Various methods of removing trace constituents and contaminantsfrom aqueous systems with the aid of adsorbents are known.

[0021] DE-A 3 800 873 describes an adsorbent based on porous materialssuch as e.g. hydrophobed chalk with a fine to medium grain size toremove contaminants from water.

[0022] DE-A 3 703 169 discloses a process for the production of agranulated filter medium to treat natural water. The adsorbent isproduced by granulating an aqueous suspension of kaolin with addition ofpowdered dolomite in a fluidised bed. The granules are then baked at 900to 950° C.

[0023] A process for the production and use of highly reactive reagentsfor waste gas and waste water purification is known from DE-A 40 34 417.Mixtures consisting of Ca(OH)₂ with additions of clays, stone dust,entrained dust and fly ashes, made porous and having a surface area ofapprox. 200 m²/g, are described here.

[0024] The cited processes and the contacts used therein have the shareddisadvantage that the component responsible in each case for theselective adsorption of constituents of the media to be cleaned, inother words the actual adsorbent, must be supplemented with largequantities of additives to enable it to be shaped into granules. Thissignificantly reduces the binding capacity for the water contaminants tobe removed. Moreover, subsequent reprocessing or reuse of the materialis problematic since the foreign substances used as binders first haveto be separated out.

[0025] DE-A 4 214 487 describes a process and a reactor for the removalof impurities from water. The medium flows horizontally through afunnel-shaped reactor, in which finely divided iron hydroxide inflocculent form is used as a sorption agent for water impurities. Thedisadvantage of this process lies in the use of the iron hydroxide inflocculent form, which means that because there is little difference indensity between water and iron hydroxide, a reactor of this type can beoperated at only very low flow rates and there is a risk of the sorptionagent, which is possibly already loaded with contaminants, beingdischarged from the reactor along with the water.

[0026] JP-A 55 132 633 describes granulated red mud, a by-product ofaluminium production, as an adsorbent for arsenic. This consists ofFe₂O₃, Al₂O₃ and SiO₂. No mention is made of the stability of thegranules or of the granulation process. A further disadvantage of thisadsorbent is the lack of consistency in the composition of the product,its unreliable availability and the possible contamination of thedrinking water with aluminium. Since aluminium is suspected ofencouraging the development of Alzheimer's Disease, contamination withthis substance in particular is to be avoided.

[0027] DE-A 19 826 186 describes a process for the production of anadsorbent containing iron hydroxide. An aqueous polymer dispersion isincorporated into iron hydroxide in water-dispersible form. This mixtureis then either dried until it reaches a solid state and the solidmaterial then comminuted mechanically to the desired shape and/or sizeor the mixture is shaped, optionally after a preliminary drying stage,and a final drying stage then performed, during which a solid state isachieved. In this way a material is obtained in which the iron hydroxideis firmly embedded in the polymer and which is said to display a highbinding capacity for the contaminants conventionally contained in wastewaters or waste gases.

[0028] The disadvantage of this process lies in the use of organicbinders, which further contaminate the water to be treated due toleaching and/or abrasion of organic substances. Furthermore, thestability of the adsorbent composite is not guaranteed in extended use.Bacteria and other microorganisms can also serve as a nutrient mediumfor an organic binder, presenting a risk that microorganisms maypopulate the contact and thereby contaminate the medium.

[0029] The presence of foreign auxiliary substances, which are requiredfor the manufacture of the adsorbents, during reprocessing, recycling orreuse of used adsorbents is disadvantageous in principle because thereuse of pure substances is less problematic than is the case with mixedsubstances. For example, polymeric binders are disadvantageous when ironoxide-based adsorption materials are reused as pigments for concretecoloration because these binders inhibit dispersion of the pigment inliquid concrete.

[0030] DE-A 4 320 003 describes a process for the removal of dissolvedarsenic from ground water with the aid of colloidal or granulated ironhydroxide. Where fine, suspended iron(III) hydroxide products are used,it is recommended here that the iron hydroxide suspension be placed infixed-bed filters filled with granular material or other supports havinga high external or internal porosity. This process likewise has thedisadvantage that, relative to the adsorbent “substrate+iron hydroxide”,only low specific loading capacities are achievable. Furthermore, thereis only a weak bond between substrate and iron hydroxide, which meansthat there is a risk of iron hydroxide or iron arsenate being dischargedduring subsequent treatment with arsenic-containing water. Thispublication also cites the use of granulated iron hydroxide as anadsorption material for a fixed-bed reactor. The granulated ironhydroxide is produced by freeze conditioning (freeze drying) of ironhydroxide obtained by neutralisation of acid iron(III) salt solutions attemperatures of below minus 5° C. This production process is extremelyenergy-intensive and leads to heavily salt-contaminated waste waters.Moreover, as a result of this production process only very smallgranules with low mechanical resistance are obtained. When used in afixed-bed reactor, this means that the size spectrum is significantlyreduced by mechanical abrasion of the particles during operation, whichin turn results in finely dispersed particles of contaminated oruncontaminated adsorption agent being discharged from the reactor. Afurther disadvantage of these granules lies in the fact that theadsorption capacity in respect of arsenic compounds is reducedconsiderably if the granules lose water, by being stored dry forextended periods for example.

[0031] Adsorbent/binder systems obtained by removing a sufficientlylarge amount of water from a mixture of (a) a crosslinkable binderconsisting of colloidal metal or non-metal oxides, (b) oxidic adsorbentssuch as metal oxides and (c) an acid such that components (a) and (b)crosslink to form an adsorbent/binder system, are known from U.S. Pat.No. 5,948,726. According to the embodiments, colloidal alumina oraluminium oxide is used as binder.

[0032] The disadvantage of these compositions lies in the need to useacid in their production (column 9, line 4) and in the fact that theyare not pure but heterogeneous substances, which is undesirable both forthe production, regeneration, removal and permanent disposal of suchadsorbents, e.g. on a waste disposal site. The scope of disclosure ofthis publication is also intended to include adsorbents that aresuitable for the adsorption of arsenic; specific examples are notprovided, however. Aluminium oxide is known to be significantly inferiorto iron oxides in regard to force of adsorption for arsenic.

[0033] Continuous adsorbers, which are commonly grouped together inparallel for operation, are preferably used for water treatment. To freedrinking water from organic impurities, for example, such adsorbers arefilled with activated carbon. At peak consumption times, the availableadsorbers are then operated in parallel to prevent the flow rate fromrising above the maximum permitted by the particular arrangement. Attimes of lower water consumption, individual adsorbers are taken out ofoperation and can be serviced, for example, whereby the adsorptionmaterial is subjected to special loads, as described in greater detailbelow.

[0034] The use of granules, which can be produced by compacting e.g.powdered iron oxide using high linear forces, has also already beenconsidered. Such granules have already been described as a means ofhomogeneously colouring liquid concrete. The use of high linear forcesfor compacting is extremely expensive and energy-intensive, and thestability of the compacted materials is inadequate for extended use inadsorbers. The use of such materials in adsorbers, for example,particularly continuous models, for water purification is therefore ofno interest. During maintenance or cleaning of adsorber plants byback-flushing in particular (see below), such granules lose largeamounts of substance due to the associated agitation. The abradedmaterial renders the waste water from back-flushing extremely turbid.This is unacceptable for a number of reasons: firstly, adsorptionmaterial, which is heavily laden with impurities and therefore toxicafter extended use, is lost. Secondly, the stream of waste water isladen with abraded material, which can sediment, damaging piping systemsand ultimately subjecting the waste treatment plant to undesirablephysical and toxicological stresses, to name but a few reasons.Preferably the abrasion should be below 20% by weight, more preferablybelow 15% by weight, 10% by weight or most preferably below 5% by weightaccording to the method described in the examples of the presentinvention.

[0035] An object underlying the present invention was therefore toprovide a filtration unit for the removal of arsenic and heavy metalsfrom drinking, process, mineral, garden pond, agricultural, holy andmedicinal water using iron oxyhydroxide or iron oxide particles as acontact or adsorbent/catalyst, which guarantees a high degree of removalof the dissolved contaminants due to the adsorption capacity of thepacking medium, which at the same time withstands the mechanical andhydraulic stresses in the adsorber housing and which for safety reasons,due to the filter performance of built-in filters, additionally preventsthe discharge of suspended impurities or abraded material from theadsorbent, possibly laden with contaminants.

[0036] This complex object is achieved by the contacts oradsorbents/catalysts according to the invention, their preparation,their use and units filled therewith.

SUMMARY OF THE INVENTION

[0037] The invention relates to a filtration unit suitable for thethrough-flow of a fluid medium for the removal of a contaminant from thefluid medium comprising a cartridge housing (4), which comprises avessel having a centrally positioned inlet pipe (6), flat filter layers(3), (10), a cover ensuring the inflow (1) and outflow (12) of themedium, together with a base part (9), wherein the cartridge housing isfilled at least partially some particles prepared from fine-particleiron oxide and/or iron oxyhydroxide having a BET surface area of 50 to500 m²/g.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The drawings show different embodiments of the invention.

[0039]FIG. 1a. describes an adsorber tank with iron hydroxide adsorbent.

[0040]FIG. 1b. describes a tapered adsorber tank with iron hydroxideadsorbent.

[0041]FIG. 2a. describes an adsorber tank with iron(oxy)hydroxide-containing adsorber cartridge and housing.

[0042]FIG. 2b. describes an adsorber tank operated in the reversedirection (cf. FIG. 2a).

[0043]FIG. 3. describes a filter cartridge housing with iron(oxy)hydroxide adsorbents.

[0044]FIG. 5. describes a bag filter with iron hydroxide granules.

DETAILED DESCRIPTION OF THE INVENTION

[0045] The cartridge housing can optionally contain iron oxide pigmentswith BET surface areas below the above limits, whereby the maximumcontent of these is such that the resistance of the charge to the forcesexerted upon it by the flowing medium is sufficiently great that thestress exerted on the charge by the flowing medium does not lead to anundesirable abrasion of the charge material.

[0046] The invention also relates to a process for the production ofparticles from fine-particle iron oxide and/or iron oxyhydroxidecomprising the steps of producing an aqueous suspension of fine-particleiron oxides and/or iron oxyhydroxides having a BET surface area of 50 to500 m²/g, removing the water and dissolved constituents by either

[0047] I) a) first removing only the water from the suspension, b)introducing the residue thus obtained in water, c) filtering thematerial obtained, d) washing the residue, and e) either e1) completelydehydrating the filter cake obtained as residue and comminuting thefilter cake to the desired shape and/or size or e2) partiallydehydrating it to obtain a paste, shaping the paste and subsequentlyadditionally drying the paste until a pellet is obtained, or

[0048] II) a) filtering the suspension, b) washing the residue, c)either c1) completely dehydrating the filter cake obtained as residue inthe form of a solid to semisolid paste and then comminuting the materialthus obtained to the desired shape and/or size or c2) partiallydehydrating it to obtain a paste, shaping the paste, followed bysubsequent additional drying until a pellet is obtained.

[0049] In this process the pellet can be further subjected to a furthercomminution by grinding or rough grinding.

[0050] In this process the BET surface area can be 80 to 200 m²/g.

[0051] In this process the water can be removed by evaporation.

[0052] In this process the residue can be washed until it is low insalts.

[0053] In this process the residue can be washed until it is free fromsalts.

[0054] In this process the iron oxides and/or iron (oxy)hydroxides canbe a commercial pigment.

[0055] In this process the iron oxides and/or iron (oxy)hydroxides canbe a transparent pigment.

[0056] In this process the iron oxide and/or iron oxyhydroxide cancomprise Fe(OH)₂.

[0057] The object of the invention is achieved by a filtration unitconsisting of a housing made from plastic, wood, glass, ceramics, metalor a composite material, provided with inlet and outlet openings. Simpleexemplary embodiments are illustrated by FIGS. 1a and 1 b. Thesehousings are described in detail in DE-A 19 816 871. The inlet andoutlet openings are separated from the actual housing chamber, whichcontains a bed of the iron oxyhydroxide adsorption medium, by flatfilter units which cover them. The fluid to be treated thus passessuccessively through the first flat filter layer, the adsorbentparticles, the second flat filter layer and the outlet opening. Thehousing chamber can be entirely or partially filled with the adsorbentparticles. The housing chamber is preferably conical or pyramidal, butmay also be obtained in a cylindrical, spherical, cuboid or spirallywound form. By tapering the housing chamber (see FIG. 1b), thefiltration can be performed in any of the layers, for example,preventing the formation of a bypass between the bed of adsorbentparticles through which the fluid to be filtered can pass unhinderedwithout adsorption. Filling the housing chamber with a bed of adsorbentparticles taking up between 97 and 99% of the volume of the housingensures a high flow rate of the fluid to be cleaned, since the flow ofliquid meets a low resistance due to the resistance of the adsorbentgranules.

[0058] In preferred embodiments of the invention, the housing chambertakes the form of a truncated cone or truncated pyramid in the taperedsections.

[0059] Depending on the area of application, a variety of materials areshown for the flat filter layers, e.g. in DE-A 19 816 871.

[0060]FIG. 2a and 2 b show an improved embodiment of an adsorber tank.They both illustrate the domestic filter module in longitudinal section.

[0061] The adsorber housing (4) with the iron oxyhydroxide adsorbentmaterial (5) with filter plates positioned top (3) and bottom (10) atthe front and a centrally positioned inlet pipe (6) can be isolated as aunit by means of a screw fitting with the cover (13) at the upper endand a screw fitting with the base cap (9) at the lower end byunfastening the screw fittings. When the cartridge is exhausted, a newone can be inserted and the base and cover plate cleaned. At the upperend the inlet pipe (6) is firmly attached to the feed nozzle (2) duringuse by means of a suitable sealing ring. The inlet pipe can be removedfrom the cartridge housing and inserted in a new, fresh cartridgehousing. The incoming liquid flows through it directly onto a strainerbasket (7), which prefilters suspended matter, algae and the like andretains it at the entrance to the actual adsorber cartridge, preventingthe adsorbent material from caking or agglutinating. The strainer (7)serves to distribute the incoming stream of liquid uniformly in the basechamber, is therefore preferably conical, i.e. in the form of atruncated cone, and completely encloses the inlet pipe and is fixed bothto this and to the surrounding filter plate (10) by means of loosesealing rings. The straining cloth can be made from standard fine-meshedfilter materials, e.g. from plastic, natural material or metal.

[0062] The screwed-on base part (9) can additionally contain a suitablefilter material or filter cloth (8), which can be selected according tothe type and quantity of the anticipated suspended material. With largequantities of solid impurities, the strainer (7) and the filter cloth(8) can easily be removed and cleaned by unscrewing the base part. Thefilter plate (10), which can consist of fine-pored ceramics, separatesthe base chamber (9) from the contact chamber with the iron oxyhydroxidegranules (5), preventing adsorbent material from entering the basechamber and prefiltered material from entering the contact chamber. Asthe water to be cleaned rises up through the contact room with the ironoxyhydroxide adsorbent, the contaminants to be removed are removed byphysisorption and/or chemisorption at the adsorbent material. Anadditional filter plate at the upper end of the cartridge housingprevents any adsorbent material from entering the outlet (12). Elevatedwater pressure or extended operating life of the adsorber tank can causefines to abrade from the adsorbent material and pass through the filterplate (3). To prevent these fines (which are laden with contaminants)from entering the outlet, filter material or filter cloth (11), whichretains the fines, is embedded inside the cover (13).

[0063] The filter layers (3) and (10) also serve to distribute the fluiduniformly in the adsorber chamber (5) and to collect it together againwhen it emerges.

[0064] The clean water, freed from impurities and contaminants, leavesthe adsorber tank via the discharge nozzle (12).

[0065] The cover (13) can also include a valve to release the gasesentrained during operation for the first time (e.g. air contained in thecartridge housing).

[0066] Depending on the application, it can be advantageous to operatethe adsorber tank described above in the reverse sequence (FIG. 2b).This means that the water to be cleaned now passes from the feed nozzle(1) directly onto the prefilter (11), which retains suspended matter andforeign material, then passes through the filter plate (3), enters thecontact chamber, where the dissolved contaminants are adsorbed on theadsorbent material, passes through the cartridge base plate (10) intothe base chamber (9), which may contain embedded filter material (8) toretain abraded adsorbent material, whereby the strainer basket (7)provides additional filtration functions, such that the cleaned waterleaves the adsorber tank through the opening (1) via the outlet pipe (6)and the discharge nozzle.

[0067]FIG. 4 illustrates a simpler embodiment, which neverthelessoperates according to the principle described above. It shows theadsorber tank, which contains the adsorbent granules according to theinvention, and in which the adsorber cartridge forms a unit.

[0068] Naturally other embodiments and designs resembling the structuredescribed and operating by the methods described are possible inprinciple, i.e. containing an inlet and outlet opening for water andiron oxide and/or iron (oxy)hydroxide as adsorbent media.

[0069]FIG. 5 illustrates a filter bag which, when filled with adsorbentgranules, can be fed to a body of water to be cleaned in order to removethe contaminants contained within it by adsorption.

[0070] Filter bags and extraction thimbles, for example, are known inmany forms and designs for the preparation of hot infusions,particularly tea. DE-A 839 405 describes a folding bag of this type, forexample, such as is used to prepare tea and the like. A special foldingtechnique, by means of which a dual chamber system is formed, ensures athorough mixing of the eluent with the substance to be extracted.

[0071] Conversely, however, iron oxides or iron (oxy)hydroxides inpowdered, finely granulated or coarsely granulated form can be embeddedin semipermeable bags or sachets having a filtering action (such as thefolding bag described above, for example), and these packages fed to thebody of water to be cleaned in order to remove the contaminants from thewater by adsorption on the adsorbent material after a certain contacttime (see FIG. 5). The iron oxides and/or iron (oxy)hydroxides withstandthe mechanical and hydraulic stresses in the filter bag on the one handand on the other hand the filter performance of the filter membraneprevents any fines from the adsorbent caused by abrasion from enteringthe water to be cleaned.

[0072] Common to the various embodiments of the present invention is thefact that iron hydroxide or iron oxyhydroxide in finely granulated,coarsely granulated or powdered form is embedded in housings having afiltering action, and the liquid to be cleaned is allowed to flowthrough the filter housing or the filter pack is fed to the liquid to becleaned, thereby ensuring adsorption of the contaminants.

[0073] To prepare the granules according to the invention, an aqueoussuspension of fine-particle iron oxyhydroxides and/or iron oxides isfirst produced according to the prior art. The water and constituentsdissolved within it can be removed from this in two different ways:

[0074] Method 1

[0075] For applications in which lower demands are made of themechanical strength of the granules/contacts, only the water is removedinitially, e.g. by evaporation. A residue is obtained which in additionto the fine-particle iron oxide and/or hydroxide also contains theentire salt content. This residue is redispersed in water after beingdried, for which purpose only relatively little shear force needs to beapplied. This suspension is then filtered and the residue washed untilit is substantially free from salts. The filter cake obtained as residueis a solid to semisolid paste which generally has a water content ofbetween 10 and 90 wt. %.

[0076] This can then be completely or partially dehydrated, and thematerial thus obtained can then be comminuted to the desired shapeand/or size. Alternatively the paste or filter cake, optionally afterpredrying to achieve a sufficiently solid state, can undergo shapingfollowed by (additional) drying until a pellet state is achieved. Thesubsequent application of the granules determines the preferredprocedure to be followed for their production, which can be determinedby the person skilled in the art in the particular field of applicationby means of simple preliminary orienting experiments. Both the directlydried filter cake and the dried shaped bodies can then be used ascontact or adsorbent.

[0077] Method 2

[0078] For applications in which higher demands are made of themechanical strength of the granules/contacts, the suspension is filteredand the residue washed until it is substantially free from salts. Thefilter cake obtained as residue is a solid to semisolid paste. This canthen be completely or partially dehydrated, and the material thusobtained can then be comminuted to the desired shape and/or size.Alternatively the paste or filter cake, optionally after predrying toachieve a sufficiently solid state, can undergo shaping followed by(additional) drying until a pellet state is achieved. The subsequentapplication of the granules determines the preferred procedure to befollowed for their production, which can be determined by the personskilled in the art in the particular field of application by means ofsimple preliminary orienting experiments. Both the directly dried filtercake and the dried shaped bodies can then be used as contact oradsorbent.

[0079] Although the products obtained according to method 1 are lessmechanically resistant, filtration can be performed more easily andquickly. The fine-particle pigments isolated in this way can moreover beincorporated very easily into paints and polymers, for example, becauseconsiderably less shear force is required than is needed to incorporatethe fine-particle pigments obtained according to method 2.

[0080] The fine-particle iron oxide and/or iron oxyhydroxide used has aparticle size of up to 500 nm, preferably up to 100 nm, particularlypreferably 4 to 50 nm, and a BET surface area of 50 to 500 m²/g,preferably 80 to 200 m²/g.

[0081] The primary particle size was determined by measurement fromscanning electron micrographs, e.g. at a magnification of 60000:1(instrument: XL30 ESEM FEG, Philips). If the primary particles areneedle-shaped, as in the (α-FeOOH phase for example, the needle widthcan be given as a measurement for the particle size. Needle widths of upto 100 nm, but mainly between 4 and 50 nm, are observed in the case ofnanoparticle α-FeOOH particles. α-FeOOH primary particles conventionallyhave a length:width ratio of 5:1 to 50:1, typically of 5:1 to 20:1. Thelength:width ratio of the needle shapes can be varied, however, bydoping or by special reaction processes. If the primary particles areisometric, as in the α-Fe₂O₃, γ-Fe₂O₃, Fe₃O₄ phases for example, theparticle diameters can quite easily also be below 20 nm.

[0082] By mixing nanoparticle iron oxides or iron (oxy)hydroxides withpigments and/or Fe(OH)₃, the presence of the cited pigment or nucleusparticles in their known particle morphology, held or glued together bythe nanoparticle nucleus particles or the amorphous Fe(OH)₃ polymer, canbe detected on the scanning electron micrographs.

[0083] Products obtainable by methods 1) or 2) can then be comminutedfurther, for example by rough grinding or grinding. However, since theproducts reduce in size autogenously on first coming into contact withwater, for example when a freshly charged adsorber unit is first filledwith water, this will generally be unnecessary.

[0084] Granulation of a semi-wet paste has proven effective as anothermethod of producing granules. Here pellets or strands are formed from asemi-wet paste, e.g. using a simple perforated metal sheet, a roll pressor an extruder, and either dried immediately or additionally shaped intoa spherical or granular form by means of a spheroniser. The still wetspherules or granules can subsequently be dried to any moisture contentwhatsoever. A residual moisture content of <50% is recommended toprevent the granules from agglomerating. A spherical shape of this typecan be advantageous for use in fixed-bed adsorbers due to the improvedpacking in the adsorber vessel that is obtained in comparison withrough-ground granules or pellets in strand form.

[0085] The filtration performance of the suspensions can generally beimproved by the use of conventional filtration-improving measures, suchas are described for example in Solid-Liquid Filtration and SeparationTechnology, A. Rushton, A. S. Ward, R. G. Holdich, 2nd edition 2000,Wiley-VCH, Weinheim, and in Handbuch der IndustriellenFest/Flüssig-Filtration, H. Gasper, D. Öchsle, E. Pongratz, 2nd edition2000, Wiley-VCH Weinheim. Coagulants can thus be added to thesuspensions, for example.

[0086] Iron carbonates can also be used in addition to or in place ofthe iron oxyhydroxides.

[0087] The products according to the invention can undergo drying inair, and/or in vacuo, and/or in a drying oven and/or on belt dryers orby spray drying, preferably at temperatures from −25 to 250° C.,particularly preferably at 60 to 120° C.

[0088] The products according to the invention preferably have aresidual water content of less than 20 wt. %.

[0089] It was found that the pellets or granules obtained in this wayhave a high binding capacity for contaminants contained in water,liquids or gases and they additionally have an adequately highresistance to flowing media in terms of mechanical or hydraulicstressing.

[0090] It is particularly surprising that during drying, fine-particleiron oxyhydroxides or iron oxides having large specific surface areassolidify into very hard agglomerates, which without the addition ofbinders have a high mechanical abrasion resistance and high hydraulicresistance to contact with flowing water, and which have a high bindingcapacity for the contaminants and trace constituents contained in thewater.

[0091] Transparent iron oxyhydroxide pigments, for example, havingspecific surface areas of over 80 m² are suitable for the use accordingto the invention of fine-particle iron oxyhydroxides. Correspondinglyfine-particle iron oxide pigments, preferably haematite, magnetite ormaghemite, can also be used, however.

[0092] The production of yellow fine-particle iron oxyhydroxide pigments(e.g. goethite) in the acid or alkaline pH range, known as acid oralkaline nuclei, is prior art. The production of other fine-particleiron oxide or iron oxyhydroxide pigments is also prior art. Suchpigments can contain structures based on α, β, γ, δ, δ′, ε phases and/orFe(OH)₂ and mixed and intermediate phases thereof. Fine-particle yellowiron oxyhydroxides can be calcined to fine-particle red iron oxides.

[0093] The production of transparent iron oxides and iron oxyhydroxidesis known e.g. according to DE-A 2 603 050 from BIOS 1144, p. 29 to 33 orfrom FIAT 814, p. 1 to 26.

[0094] Fine-particle yellow iron oxihydroxide pigments are generallysynthesised by precipitating iron(II) hydroxides or carbonates fromcorresponding iron(II) salt solutions such as e.g. FeSO₄, FeCl₂ in pureform or as pickling solutions in the acid or alkaline pH range, followedby oxidation to iron(III) oxihydroxides (see inter alia G. Buxbaum,Industrial Inorganic Pigments, VCH Weinheim, 2nd edition, 1998, p.231ff). Oxidation of the divalent to the trivalent iron is preferablyperformed with air, whereby intensive aeration is advantageous.Oxidation with H₂O₂ also leads to fine-particle iron oxyhydroxides. Thetemperature chosen for precipitation and oxidation should be as low aspossible in order to obtain very fine-particle yellow pigments. It ispreferably between 15° C. and 45° C. NaOH is preferably used as alkalineprecipitant. Other precipitants, such as KOH, Na₂CO₃, K₂CO₃, CaO,Ca(OH)₂, CaCO₃, NH₃, NH₄OH, MgO and/or MgCO₃, can also be used, however.

[0095] To steer the precipitated pigments in the direction of theextremely fine-particle character that is required, the precipitations,e.g. of yellow α-FeOOH as described in patents U.S. Pat. Nos. 2,558,303and 2,558,304, are performed in the alkaline pH range with alkalicarbonates as precipitants, and modifiers such as SiO₂, zinc, aluminiumor magnesium salts, hydroxycarbonic acids, phosphates and metaphosphatesare generally added. Products produced in this way are described in U.S.Pat. No. 2,558,302. Such nucleus modifiers do not interfere with thesubsequent reprocessing, recycling or any other use of the adsorbentsaccording to the invention. In the case of precipitation processes in anaqueous medium, it is known that precipitations in an alkalineenvironment lead to less solidly agglomerated powders than those in anacid environment.

[0096] DE-A 4 235 945 reports on the production of fine-particle ironoxides using a precipitation method in the acid pH range and withoutmodifiers.

[0097] DE-A 4 434 669 describes a process by which highly transparentyellow, chemically pure iron oxide pigments can be produced by secondarytreatment thereof with sodium hydroxide solution.

[0098] DE-A 4 434 972 reports on highly transparent, yellow iron oxidepigments in the α-FeOOH modification having a specific surface area ofover 100 m²/g and high temperature resistance.

[0099] DE-A 4 434 973 describes highly transparent yellow iron oxidepigments, which are produced by means of the process steps of nuclearprecipitation in the acid pH range, nuclear oxidation, nuclearmaturation and pigment formulation.

[0100] Red, transparent iron oxide pigments obtained by calcining fromyellow, transparent iron oxide pigments are known from DE-A 4 434 668and DE-A 4 235 946.

[0101] By preparing diverse phases of iron oxyhydroxides in pure form orin any mixture from iron(II) salt solutions using the knownprecipitation and oxidation reactions, separating the resultant ironoxyhydroxides out of the suspension, optionally after a secondarytreatment, by filtering the salt solution and washing them until theyare largely free from salts, preferably down to a residual conductivityof <5 mS/cm, then drying the solid or semisolid filter cake just as itis or optionally after mechanical shaping until it achieves a solidstate, a mechanically highly resistant material displaying a highbinding capacity for the contaminants conventionally contained in wastewaters or waste gases is obtained.

[0102] Drying is conveniently performed at temperatures of up to 250° C.The material can also be vacuum or freeze dried. The particle size ofthe material can be freely selected but is preferably between 0.2 and 40mm, particularly preferably between 0.2 and 20 mm. This can be achievedby shaping the semisolid, pasty filter cake mechanically before dryingin a granulation or pelletising plant or in an extruder to form shapedbodies whose size is in the range between 0.2 and 20 mm, with subsequentdrying in the air, on a belt dryer or in a drying oven, and/or bymechanical comminution to the desired particle size after drying.

[0103] The products described, the process for their production andtheir use represent an improvement over the prior art. In contrast tothose based on coarse-particle iron oxyhydroxides and/or oxides, thegranules according to the invention based on fine-particle iron(oxy)hydroxides and/or oxides can be subjected to much higher stressesand therefore display a much greater abrasion resistance to mechanicaland hydraulic stressing. They can be used directly as such. When used inadsorber plants for water purification, for example, there is no needeven for comminution or rough grinding of the crude dry substanceinitially obtained from filter cakes or extruders, since the coarsepellets break down independently on contact with water. This results ina random particle-size distribution, but no particles of such a sizethat they are discharged from the adsorber to any significant extent bythe flowing medium.

[0104] There is absolutely no need for a separate granulation process,such as would be necessary when using conventional iron oxyhydroxides inthe form of (flowable) powders, either with the aid of foreign bindersor using extremely high linear forces during compacting.

[0105] According to the invention, the suspensions of fine-particle ironoxyhydroxides or iron oxides can also be supplemented with conventionalpowdered iron oxyhydroxides or iron oxides. The quantities in each caseare determined by the properties of these powdered iron oxyhydroxides oriron oxides and by the requirements of the product according to theinvention in terms of its mechanical stability and abrasion resistance.Although the addition of powdered pigments will generally reduce themechanical strength of the products according to the invention,filtration of the fine-particle suspensions is made easier. The personskilled in the art and practising in the particular field of applicationwill be able to determine the optimum mixing ratio for the intendedapplication by means of a few orienting experiments.

[0106] A quantity of aqueous salts of Fe³⁺, Al³⁺, Mg²⁺, Ti⁴⁺ or mixturesthereof corresponding to the NaOH excess can be added to the suspensionsof the alkaline fine-particle nuclei until sufficiently poorly solubledeposits of Fe(OH)₃, Al(OH)₃, Mg(OH)₂, TiO(OH)₂ or ageing products anddehydrated secondary products thereof are precipitated onto thesuspended iron oxide and/or iron (oxy)hydroxide particles. Conversely,the poorly soluble deposits of Fe(OH)₃, Al(OH)₃, Mg(OH)₂, TiO(OH)₂ orageing products and secondary products thereof can be precipitated ontothe iron oxide or iron (oxy)hydroxide particles suspended in Fe³⁺, Al³⁺,Mg²⁺, Ti⁴⁺ solutions by the addition of alkalis, such as e.g. NaOH,Ca(OH)₂, KOH, CaCO₃, Na₂CO₃, K₂CO₃, NH₄OH. The aluminium oxide oraluminium (oxy)hydroxide can also be precipitated from an aluminatesuspension (e.g. NaAlO₂) onto the iron oxide and/or iron (oxy)hydroxideparticles.

[0107] The initially amorphous Fe(OH)₃ or Al(OH)₃ produced mature overtime, to the FeOOH or AlOOH phase, for example. This ensures that thesodium hydroxide solution used in excess to produce the alkaline nucleusis completely used up. The materials thus obtained also display largespecific surface areas. Just like the nanoparticle iron oxyhydroxidesdescribed above, the material is extremely suitable for use in adsorberssince it possesses a high resistance to mechanical loading in additionto a high adsorption capacity.

[0108] The granules according to the invention are particularlypreferably used in the cleaning of liquids, especially for the removalof heavy metals. A preferred application in this industrial field is thedecontamination of water, particularly of drinking water. Particularattention has recently been paid to the removal of arsenic from drinkingwater. The granules according to the invention are extremely suitablefor this purpose, since levels that not only meet but actually fallbelow even the lowest limiting values set by the US authority the EPAcan be achieved using the granules according to the invention.

[0109] To this end the granules can be used in conventional adsorberunits, such as are already used with a charge of activated carbon, forexample, to remove other types of contaminants. Batchwise operation, incisterns or similar containers for example, optionally fitted withagitators, is also possible. However, use in continuous plants such ascontinuous-flow adsorbers is preferred.

[0110] Since untreated water to be processed into drinking waterconventionally also contains organic impurities such as algae andsimilar organisms, the surface of adsorbents, especially the outersurface of granular adsorbents, becomes coated during use with mostlyslimy deposits, which impede or even prevent the inflow of water andhence the adsorption of constituents to be removed. For this reasonadsorber units are periodically back-flushed with water, a process whichis preferably performed at times of low water consumption (see above) onindividual units that have been taken out of service. The adsorbent iswhirled up and the associated mechanical stress to which the surface issubjected causes the undesirable coating to be removed and dischargedagainst the direction of flow during active operation. The wash water isconventionally sent to a sewage treatment plant. The adsorbentsaccording to the invention have proven to be particularly effective inthis process, since their high strength enables them to be cleanedquickly without suffering any significant losses of adsorption materialand without the back-flush water sent for waste treatment being rich indischarged adsorption material, which is possibly already highlycontaminated with heavy metals.

[0111] The impurities that could block the adsorber cartridge areretained by a suitable prefilter and afterfilter.

[0112] Material abrasion is minimised by the resistance according to theinvention of the granules and by suitable packing of the adsorbergranules.

[0113] Spraying granules of iron oxyhydroxide adsorbent having aparticle size <250μ have proven to be particularly favourable becausethey lead to a particularly good packing density.

[0114] Since the granules according to the invention are free fromforeign binders, the material is comparatively easy to dispose of afteruse. For instance, the adsorbed arsenic can be removed by thermal orchemical means in special units, for example, resulting in an iron oxidepigment as a pure substance which can either be recycled for use in thesame application or supplied for conventional pigment applications.Depending on the application and legal regulations, the content of theadsorber can also be used without prior removal of the heavy metals, forexample as a pigment for colouring durable construction materials suchas concrete, since the heavy metals removed from the drinking water arepermanently immobilised in this way and taken out of the hydrologicalcycle.

[0115] The invention therefore also provides water treatment plants orwaterworks in which units filled with the granules according to theinvention are operated, and processes for the decontamination of waterby means of such units, as well as such units themselves.

[0116] For many applications, particularly those in which a maximummechanical strength is not required of the granules, the addition ofpowdered pigments during production of the granules according to theinvention is a preferred embodiment.

[0117] Thus, for example, up to 40 wt. % of commercial goethite (e.g.Bayferrox® 920, Bayer AG, Leverkusen DE) can be added to a nucleussuspension according to example 2 of the present application if thegranules obtained according to the invention are to be used for theremoval of arsenic from drinking water in adsorbers with a through-flowof water.

[0118] The BET specific surface area of the products according to theinvention is determined by the carrier gas process (He:N₂=90:10) usingthe single-point method, according to DIN 66131 (1993). The sample isbaked for 1 h at 140° C. in a stream of dry nitrogen before measurement.

[0119] In order to measure the adsorption of arsenic(III) andarsenic(V), 3 litres of an aqueous solution of NaAsO₂ or Na₂HAsO₄, eachwith the specified concentration of approx. 2-3 mg/l arsenic, aretreated with 3 g of the sample to be tested in a 5 litre PE flask for aspecific period and the flask moved on rotating rollers. The adsorptionrate of As ions on iron hydroxide over this specific period, e.g. onehour, is stated as mg(As^(3+/5+))/g(FeOOH)·h, calculated from thebalance of the As^(3+/5+) ions remaining in solution.

[0120] The adsorption of Sb³⁺, Sb⁵⁺, Pb²⁺, Hg²⁺, Cr⁶⁺ or Cd²⁺ ions ismeasured in the same way, whereby the desired concentrations areestablished by dissolving appropriate amounts of Sb₂O₃, KSb(OH)₆, PbCl₂,NaCrO₄ or CdCl₂ in H₂O and adjusting the pH value to 7-9.

[0121] The As, Sb, Cd, Cr, Hg or Pb contents of the contaminated ironoxyhydroxide or of the solutions are determined using mass spectrometry(ICP-MS) according to DIN 38406-29 (1999) or by optical emissionspectroscopy (ICP-OES) according to EN-ISO 11885 (1998), withinductively coupled plasma as excitation agent in each case.

[0122] The mechanical and hydraulic abrasion resistance was assessedusing the following method: 150 ml of demineralised water were added to10 g of the granules to be tested, having particle sizes >0.1 mm, in a500 ml Erlenmeyer flask, which was rotated on a LabShaker shakingmachine (Kühner model from Braun) for a period of 30 minutes at 250 rpm.The >0.1 mm fraction was then isolated from the suspension using ascreen, dried and weighed. The weight ratio between the amount weighedout and the amount weighed in determines the abrasion value in %.

[0123] The invention is described in greater detail below by means ofexamples. The examples are intended to illustrate the process and do notconstitute a limitation.

EXAMPLES Example 1

[0124] 237 l of an aqueous iron sulfate solution with a concentration of150 g/l FeSO₄ were prepared at 24° C. 113 l of an aqueous NaOH solution(227 g/l) were then quickly added and the light blue suspension thenoxidised with 40 l of air per hour and per mol of iron for 1.5 hours.

[0125] The yellow suspension thus obtained was filtered out through afilter press and the solid washed until the residual filtrateconductivity was 1 mS/cm. The filter cake was in the form of aspreadable and kneadable paste, which was dried on metal sheets in acirculating air drying oven at 75° C. until the residual moisturecontent was 3 wt. %. The dried material was then roughly ground toproduce particle sizes of between 0.5 and 2 mm. The hard pellets thusobtained were then placed directly in an adsorber tank.

[0126] The product consisted of 100% α-FeOOH with an extremelyshort-needled habit, whereby the needles were congregated to form solidmacroscopic agglomerates. Using a scanning electron micrograph e.g. at amagnification of 60000:1, the needle widths were measured at between 15and 35 nm, the needle lengths between 150 and 350 nm. The needles wereextremely agglomerated.

[0127] The BET specific surface area was 122 m²/g. The adsorption ratefor NaAsO₂ with an original concentration of 2.3 mg (As³⁺)/l was 2.14mg(As³⁺)/g(FeOOH)·h, for Na₂HAsO₄ with an original concentration of 2.7mg (As⁵⁺)/l it was 2.29 mg(As⁵⁺)/g(FeOOH)·h.

Example 2

[0128] 800 l of an aqueous iron sulfate solution with a concentration of150 g/l FeSO₄ were prepared at 29° C. and 147 l of an aqueous NaOHsolution (300 g/l) added over 20 minutes with stirring. 2.16 kg of a 57%aqueous glycolic acid solution were then added to the grey-bluesuspension formed and oxidation performed for 7 hours with 38 l of airper hour and per mol of iron.

[0129] The dark brown suspension was filtered out through a filter pressand the solid washed until the residual filtrate conductivity was 1mS/cm. The filter cake was dried at 70° C. in a circulating air dryingoven to a residual moisture of 5%, and the very hard blackish brown dryproduct was roughly ground in a roller crusher to particle sizes of upto 2 mm. The fine fraction <0.2 mm was separated out using a screen.

[0130] An X-ray diffractogram showed that the product consisted of 100%α-FeOOH. Using a scanning electron micrograph e.g. at a magnification of60000:1, the needle widths were measured at between 15 and 20 nm, theneedle lengths between 50 and 80 nm. The particles were extremelyagglomerated. The BET specific surface area was 202 m²/g. The granulesthus obtained were placed directly in an adsorber tank with no furthertreatment.

[0131] The granules displayed an excellent adsorption performance inrespect of the contaminants contained in the flowing water anddemonstrated a high abrasion resistance, particularly when the adsorbertank is being back-flushed causing the granules to be whirled upstrongly. The abrasion value after 30 minutes was only 1%.

[0132] Adsorption performance: The adsorption rate for NaAsO₂ with anoriginal concentration of 2.4 mg (As³⁺)/l was 1.0 mg(As³⁺)/g(FeOOH)·h,for Na₂HAsO₄ with an original concentration of 2.8 mg (As⁵⁺)/l it was2.07 mg(As³⁺)/g(FeOOH)·h.

Example 3

[0133] 1.3 l of an aqueous 300 g/l NaOH solution were added to anα-FeOOH suspension obtained according to example 2 after a two-hourmaturation at 30° C. with stirring, and post-oxidation was performedsimultaneously for one hour with 190 l of air. The product was processedas described in example 2. Fine-particle needles of pure α-FeOOH with aBET specific surface area of 130 m²/g were obtained. Using a scanningelectron micrograph e.g. at a magnification of 60000:1, the needlewidths were measured at between 15 and 20 nm, the needle lengths between50 and 90 nm. The needles were extremely agglomerated. The granulesproved to be very mechanically and hydraulically resistant, and theabrasion value was only 3.9%.

[0134] Adsorption performance: The adsorption rate for NaAsO₂ with anoriginal concentration of 2.3 mg (As³⁺)/l was 1.1 mg(As³⁺)/g(FeOOH)·h,for Na₂HAsO₄ with an original concentration of 2.8 mg (As⁵⁺)/l it was1.7 mg(As³⁺)/g(FeOOH)·h.

Example 4

[0135] 306 l of an aqueous NaOH solution (45 g/l) were prepared at 31°C. and 43 l of an aqueous solution of FeCl₂ (344 g/l) quickly added withstirring, and oxidation was then performed with 60 l of air per hour andper mol Fe. The dark yellow suspension thus obtained was processed inthe same way as in example 1.

[0136] An X-ray diffractogram showed that the product consisted of 100%α-FeOOH. Using a scanning electron micrograph e.g. at a magnification of60000:1, the needle widths were measured at between 15 and 50 nm, theneedle lengths between 100 and 200 nm. The needles were extremelyagglomerated. The BET specific surface area was 132 m²/g.

[0137] The granules thus obtained were placed in an adsorber tank withno further treatment. The granules displayed an excellent adsorptionperformance in respect of the contaminants contained in the water anddemonstrated a high abrasion resistance, particularly when the adsorbertank is being back-flushed causing the granules to be whirled upstrongly. The abrasion value after 30 minutes was only 12 wt. %.

[0138] Adsorption performance: The adsorption rate for NaAsO₂ with anoriginal concentration of 2.4 mg (As³⁺)/l was 2.11 mg(As³⁺)/g(FeOOH)·h,for Na₂HAsO₄ with an original concentration of 2.7 mg (As⁵⁺)/l it was2.03 mg(As⁵⁺)/g(FeOOH)·h.

Example 5

[0139] 124 l of an aqueous NaOH solution (114 g/l) were prepared at 24°C. and 171 l of an aqueous solution of FeSO₄ (100 g/l) quickly addedwith stirring, and oxidation was then performed with 10 l of air perhour and per mol Fe. Immediately upon completion of oxidation, 56 l ofan aqueous solution of Fe₂(SO₄)₃ (100 g/l) were added and stirred for 30minutes. The yellowish brown suspension thus obtained was processed inthe same way as in example 1.

[0140] An X-ray diffractogram showed that the product consisted of 100%α-FeOOH. Using a scanning electron micrograph e.g. at a magnification of60000:1, the needle widths were measured at between 15 and 35 nm, theneedle lengths between 70 and 180 nm. The needles were extremelyagglomerated. The BET specific surface area was 131 ml/g. The abrasionvalue after 30 minutes was only 7 wt. %.

[0141] Adsorption performance: The adsorption rate for NaAsO₂ with anoriginal concentration of 2.3 mg (As³⁺)/l was 1.7 mg(As³⁺)/g(FeOOH)·h,for Na₂HAsO₄ with an original concentration of 2.7 mg (As⁵⁺)/l it was1.2 mg(As⁵⁺)/g(FeOOH)·h.

Example 6

[0142] 7905 kg FeSO₄ were measured out, dissolved with water to a volumeof 53.3 m³, the solution cooled to 14° C. and 1000 kg MgSO₄·7 H₂O addedto this solution. The prepared solution was then diluted at 14° C. with5056 kg NaOH as a solution with approx. 300 g/l and then oxidised with4000 m³/h air to a precipitation degree of >99.5%. The batch was washedon a filter press until the residual filtrate conductivity was <1000μS/cm and the paste pushed through a perforated metal plate with holediameters of 7 mm, causing it to be formed into strands. The strandswere dried on a belt dryer to a residual moisture of approx. 3%. AnX-ray diffractogram showed that the product consisted of 100% α-FeOOHwith very short needles. Using a scanning electron micrograph e.g. at amagnification of 60000:1, the needle widths were measured at between 30and 50 nm. The needle lengths could not be clearly determined as theneedles were too greatly agglomerated. The BET specific surface area was145 m²/g. The abrasion value after 30 minutes was only 6%.

[0143] Adsorption performance: The adsorption rate for NaAsO₂ with anoriginal concentration of 2.5 mg (As³⁺)/l was 1.8 mg(As³⁺)/g(FeOOH)·h,for Na₂HAsO₄ with an original concentration of 2.9 mg (As⁵⁺)/l it was1.5 mg(As⁵⁺)/g(FeOOH)·h.

Example 7

[0144] 4096 kg NaOH (as solution with approx. 300 g/l) were prepared anddiluted with water to 40 m³. 4950 kg FeSO₄ were dissolved with water toform 48.5 m³ solution, cooled to 15° C. and then pumped into theprepared NaOH over 1 h. The suspension was then oxidised with 1500 m³/hair over approx. 2 h. Approx. 2 m³ of the nucleus suspension was washedon a filter press to obtain a filtrate conductivity <1000 μS/cm, thefilter cake was dried in a drying oven at 75° C. and the dried materialroughly ground to particle sizes <1.5 mm. The fine fraction <0.5 mm wasseparated out using a screen. The material thus obtained had a BETspecific surface area of 153 m²/g and consisted of 100% α-FeOOH. Using ascanning electron micrograph e.g. at a magnification of 60000:1, theneedle widths were measured at between 15 and 35 nm, the needle lengthsbetween 50 and 100 nm. The needles were extremely agglomerated.

[0145] Adsorption performance: The adsorption rate for NaAsO₂ with anoriginal concentration of 2.7 mg (As³⁺)/l was 1.7 mg(As³⁺)/g(FeOOH)·h,for Na₂HAsO₄ with an original concentration of 2.8 mg (As⁵⁺)/l it was1.4 mg(As⁵⁺) g(FeOOH)·h.

Example 8

[0146] An aqueous solution of FeSO₄ (100 g/l) was added to 1600 g of thealkaline nucleus suspension prepared according to example 7 (2.7% FeOOH)at room temperature with stirring and simultaneous aeration with 130 l/hof air until a pH of 8 was obtained. The nucleus suspension obtained wasfiltered, washed and the filter cake dried at 75° C. and roughly groundto particle sizes of between 0.5 and 2 mm as described in example 7. Thematerial thus obtained had a BET specific surface area of 163 m²/g andaccording to the X-ray diffractogram consisted of 100% α-FeOOH. Thescanning electron micrograph, e.g. at a magnification of 60000:1, showedthat the needles were extremely agglomerated. Adsorption performance:The adsorption rate for NaAsO₂ with an original concentration of 2.7 mg(As³⁺)/l was 2.0 mg(As³⁺)/g(FeOOH)·h, for Na₂HAsO₄ with an originalconcentration of 2.7 mg (As⁵⁺)/l it was 1.9 mg(As⁵⁺)/g(FeOOH)·h, forKSb(OH)₆ (original concentration 3.0 mg (Sb⁵⁺)/l) the adsorption was 2.5mg (Sb⁵⁺)/g (FeOOH)·h, for Na₂CrO₄ (original concentration 47 μg(Cr⁶⁺)/l) 42 μg (Cr⁶⁺)/g(FeOOH)·h were adsorbed, for PbCl₂ (originalconcentration 0.94 mg (Pb²⁺)/l) 0.46 mg (Pb²⁺)/ g(FeOOH)·h wereadsorbed.

Example 9

[0147] 6.4 l of an aqueous solution of NaOH (100 g/l) were prepared at29° C. with stirring and 12.2 l of an aqueous iron(II) sulfate solution(100 g/l) were added with simultaneous introduction of air until a pH of9 was obtained. The suspension thus obtained was processed in the sameway as in example 1. The material had a BET specific surface area of 251m²/g and according to the X-ray diffractogram consisted of 100% α-FeOOH.The scanning electron micrograph shows short, stumpy needles, which areextremely agglomerated. Abrasion performance: 5%.

[0148] Adsorption performance: The adsorption rate for NaAsO₂ with anoriginal concentration of 2.7 mg (As³⁺)/l was 1.1 mg(As³⁺)/g(FeOOH)·h,for Na₂HAsO₄ with an original concentration of 2.7 mg (As³⁺)/l it was1.0 mg(As⁵⁺)/g(FeOOH)·h.

Example 10

[0149] 4096 kg NaOH (as solution with approx. 300 g/l) were measured outand diluted with water to 40 m³. 4950 kg FeSO₄ were dissolved with waterto form 48.5 m³ solution, cooled to 15° C. and then pumped into theprepared NaOH over 1 h. The suspension was then oxidised with 1500 m³/hair in approx. 2 h. 14.4 m³ FeClSO₄ solution (113.4 g/l) were added toapprox. 87 m³ of this suspension with stirring, and stirred for afurther 30 min. The batch was washed on a filter press until theresidual filtrate conductivity was <1000 μS/cm and the paste pushedthrough a perforated metal plate with hole diameters of 7 mm and formedinto strands. The strands were dried on a belt dryer to a residualmoisture of approx. 5%. The dry pellets were roughly ground to obtain aparticle size of 2 mm. The material thus obtained had a BET specificsurface area of 142 m²/g and consisted of 100% α-FeOOH. Using a scanningelectron micrograph e.g. at a magnification of 60000:1, the needlewidths were measured at between 15 and 50 nm, the needle lengths between10 and 150 nm. The needles were extremely agglomerated.

[0150] Adsorption performance: The adsorption rate for NaAsO₂ with anoriginal concentration of 2.7 mg (As³⁺)/l was 2.1 mg(As³⁺)/g(FeOOH)·h,for Na₂HAsO₄ with an original concentration of 2.8 mg (As⁵⁺)/l it was2.0 mg(As⁵⁺)/g(FeOOH)·h, for CdCl₂ (original concentration 2.7 mg(Cd²⁺)/l) the adsorption was 1.1 mg (Cd²⁺)/g(FeOOH)·h, for KSb(OH)₆(original concentration 2.6 mg (Sb⁵⁺)/l) it was 1.9 mg(Sb⁵⁺)/g(FeOOH)·h, for Sb₂O₃ (original concentration 2.3 mg (Sb³⁺)/l) itwas 2.0 mg (Sb³⁺)/g(FeOOH)·h, for Na₂CrO₄ (original concentration 2.6 mg(Cr⁶⁺)/l) it was 1.1 mg (Cr⁶⁺), for PbCl₂ (original concentration 1.6 mg(Pb²⁺)/l) it was 1.57 mg (Pb²⁺)/g(FeOOH)·h.

Example 11

[0151] 3100 kg NaOH (as solution with approx. 100 g/l) were measured outand diluted with cold water to 31 m³. The temperature of the solutionwas 26° C. 3800 kg FeSO₄ were dissolved with water to form about 38 m³solution, cooled to 13-14° C. and then pumped with stirring into theprepared NaOH. The suspension was then oxidised with 2500 m³/h air inapprox. 75 m. 18.2 m³ FeSO₄ solution (100 g/l) were added at a rate of150 l/min to this suspension with stirring and gassing. The suspensionwas filtered on a filter press and washed until the residual filtrateconductivity was <1000 μS/cm, the paste was pushed through a perforatedmetal plate and were dried on a belt dryer to a residual moisture ofless than 20%. The dry pellets were roughly ground to obtain a particlesize of less than 2 mm. The portion of the particles with less then 0.5mm was removed. The material thus obtained had a BET specific surfacearea of 145 m²/g and consisted of 100% α-FeOOH.

Example 12

[0152] 569 ml of an MgSO₄ solution (100 g/l) were added to 1 l of asuspension of Bayferrox© 920 with a solids content of 50 g/l FeOOH, then173 g of a 24% NaOH solution were added with stirring, and stirring wascontinued for a further 15 min. The yellow suspension is washed at anutsch filter to obtain a residual filtrate conductivity of 1 mS/cm, andthe filter cake dried to a residual moisture of <2% in a drying oven at75° C. The product was granulated to particle sizes of between 0.5 and 2mm and the granules used for arsenic adsorption.

[0153] An X-ray diffractogram shows that the product consists of α-FeOOHand Mg(OH)₂. The scanning electron micrograph, e.g. at a magnificationof 60000:1, shows that the α-FeOOH type needles are agglomerated orglued together by amorphous layers. The BET specific surface area was 43m²/g and therefore, compared with Bayferrox© 920 (BET approx. 15 m²/g).The abrasion value after 30 minutes was only 11%.

[0154] The adsorption rate for an aqueous NaAsO₂ solution with anoriginal concentration of 2.6 mg (As³⁺)/l was 1.2 mg(As³⁺)/g(FeOOH)·h,for an Na₂HAsO₄ solution with an original concentration of 2.7 mg(As⁵⁺)/l it was 1.5 mg(As⁵⁺)/g(FeOOH)·h.

Example 13

[0155] 46 ml of an Al₂(SO₄)₃ solution (100 g/l Al₂O₃) were added to 950g of a suspension of an alkaline nanoparticle nucleus of α-FeOOH (solidscontent: 5.26 g/l FeOOH, 1.14% NaOH) with stirring, and stirring wascontinued for a further 15 min. The brown suspension is washed at anutsch filter to obtain a residual filtrate conductivity of 1 mS/cm, andthe filter cake dried to a residual moisture of <2% in a drying oven at75° C. The product was granulated to particle sizes of between 0.5 and 2mm and the granules used for arsenic adsorption.

[0156] The X-ray diffractogram of the product indicated only α-FeOOH,which, as can be seen from the scanning electron microgram, is presentas very short and extremely agglomerated needles. The BET specificsurface area was 102 m²/g. The abrasion value after 30 minutes was only5%.

[0157] The adsorption rate for an aqueous NaAsO₂ solution with anoriginal concentration of 2.6 mg (As³⁺)/l was 2.0 mg(As³⁺)/g(FeOOH)·h,for an Na₂HAsO₄ solution with an original concentration of 2.1 mg(As⁵⁺)/l it was 1.5 mg(As⁵⁺)/g(FeOOH)·h.

Embodiment Example 14

[0158] Adsorbent granules produced according to examples 1 to 12,typically between 0.5 and 2 mm or in comminuted form are placed in acontact chamber as shown in FIG. 1 or 2. The filtration unit displays aflow rate for air as fluid of 2000 ml per minute at a pressuredifference of 0.1 bar.

What is claimed is:
 1. A unit suitable for the through-flow of a fluidmedium for the removal of a contaminant from the fluid medium comprisinga cartridge housing, which comprises a vessel having a centrallypositioned inlet pipe, flat filter layers, a cover ensuring the inflowand outflow of the medium, together with a base part, wherein thecartridge housing is filled with at least partially some particlesprepared from fine-particle iron oxide and/or iron oxyhydroxide having aBET surface area of 50 to 500 m²/g.
 2. The unit of claim 1, wherein thecartridge housing is separated from the cover and/or from the base partby means of a plug-in or screw fitting.
 3. The unit of claim 1, whereinthe inlet pipe can be removed from the cartridge housing.
 4. The unit ofclaim 1 wherein the fluid medium is a gas.
 5. The unit of claim 4,wherein the flat filter units comprise a hydrophobic membrane.
 6. Theunit of claim 5, wherein the membrane displays a pore diameter in therange from 0.2 to 0.5 μm.
 7. The unit of claim 5, wherein thehydrophobic membrane comprises polytetrafluoroethylene.
 8. The unit ofclaim 1 wherein the fluid medium comprises a liquid.
 9. The unit ofclaim 8 wherein the medium comprises an aqueous liquid.
 10. The unit ofclaim 9, wherein the flat filter units comprise a hydrophilic membrane.11. The unit of claim 10, wherein the hydrophilic membrane is a membraneadsorber.
 12. The unit of claim 9, wherein the membrane displays a porediameter in the range from 0.2 to 0.5 μm.
 13. The unit of claim 1,wherein at least one of the flat filter layers is supported on one orboth sides.
 14. The unit of claim 1, wherein the cover comprises a valvefor escaping gases.
 15. The unit of claim 1, wherein the housing chamberis in the form of a truncated cone.
 16. The unit of claim 1, wherein thecartridge housing can optionally contain iron oxide pigments with BETsurface areas below the above limits, whereby the maximum content ofthese is such that the resistance of the particles to the forces exertedupon it by the flowing medium is sufficiently great that the stressexerted on the particles by the flowing medium does not lead to anundesirable abrasion of the particles.
 17. A process for treating afluid medium in a unit of claim 1, comprising the steps of flowing themedium through the feed nozzle, into the inlet pipe, the strainerbasket, any filter material in the base chamber, the lower frittedplate, followed by the adsorbent material in the contact chamber, theupper fritted plate, the cover chamber with filter material and then theoutlet pipe via the discharge nozzle.
 18. A process of claim 17, whereinthe flow of the fluid medium is reversed.
 19. A process comprising thestep of treating a fluid medium in a unit according to claim 1, bycontacting the fluid medium with particles obtained by a process for theproduction of an adsorbent/catalyst, wherein (a) aluminium, iron,magnesium and/or titanium oxides or (oxy)hydroxides or ageing productsand dehydrated secondary products thereof are incorporated into anaqueous suspension of iron oxide and/or iron oxyhydroxide, includingFe(OH)₂ and then (b) either (b1) the suspension is dried until itreaches a solid state and the solid material then comminutedmechanically to the desired shape and/or size or (b2) the suspensionundergoes mechanical shaping, optionally in the semisolid state afterpredrying, followed by additional drying until a solid state isachieved.
 20. The process of claim 19 wherein the fluid medium compriseswater.
 21. The process of claim 19 comprising removing a heavy metal,phosphorus, antimony, beryllium, selenium, tellurium or cyano compoundfrom water.
 22. The process of claim 19 comprising removing an arseniccompound from water.
 23. A process comprising the step of treating afluid medium in a unit according to claim 1, by contacting the fluidmedium with particles obtained by a process for the production ofparticles from fine-particle iron oxide and/or iron oxyhydroxidecomprising the steps of producing an aqueous suspension of fine-particleiron oxides and/or iron oxyhydroxides having a BET surface area of 50 to500 m²/g, removing the water and dissolved constituents by either I) a)first removing only the water from the suspension, b) introducing theresidue thus obtained in water, c) filtering the material obtained, d)washing the residue, and e) either e1) completely dehydrating the filtercake obtained as residue and comminuting the filter cake to the desiredshape and/or size or e2) partially dehydrating it to obtain a paste,shaping the paste and subsequently additionally drying the paste until apellet is obtained, or II) a) filtering the suspension, b) washing theresidue, c) either c1) completely dehydrating the filter cake obtainedas residue in the form of a solid to semisolid paste and thencomminuting the material thus obtained to the desired shape and/or sizeor c2) partially dehydrating it to obtain a paste, shaping the paste,followed by subsequent additional drying until a pellet is obtained. 24.The process of claim 23 wherein the fluid medium comprises water. 25.The process of claim 23 comprising removing a heavy metal, phosphorus,antimony, beryllium, selenium, tellurium or cyano compound from water.26. The process of claim 23 comprising removing an arsenic compound fromwater.
 27. A container able to be contacted with a fluid medium, atleast partially filled with particles agglomerated from fine-particleiron oxide and/or iron oxyhydroxide wherein the particles are preparedfrom an aqueous suspension of fine-particle iron oxides and/or ironoxyhydroxides having a BET surface area of 50 to 500 m²/g, by removingthe water and dissolved constituents.